2 This Talk DeviceNet - Background DeviceNet v. Profibus DP Operation of 26/2700Operation of 2400 and 2500
3 DeviceNet Based on CAN (Controller Area Network) CAN provides deterministic “peer-to-peer” networkingDeviceNet is to CAN as Modbus is to RS485Higher Level Protocol designed for Industrial applicationsPower available on the same cabling (up to 8 amps at 24VDC)Designed by Allen Bradley as proprietary fieldbus for linking simple devices (e.g. limit switches) to a central PLCNow fully “open” and owned by ODVA (Open DeviceNet Vendors Association)
4 DeviceNet Usually uses a PLC “Master” and “Slave” devices Typically very simple networks (< 600 bytes of Input and Output for the whole network).Limited data capacity, imposed by slow maximum speed (500K), and CAN limitationsLimited physical network length (discussed later)DeviceNet can support complex peer-to-peer networks andmore complex devices. However in practice, this is almostnever done.
5 I/O Data Transfer Model Slave Input Data (To PLC)PLC RegistersTemperatureWorking SetpointLoop Output PowerAlarm Status wordSlave Output Data (From PLC)Target SetpointManual OutputApplication usesdata from inputregisters andsets values inoutput registersScanner transfers I/OData from/to PLC onceeach scan cycle for eachslave ~ 10mS cycle
6 I/O Transfer Methods“Polled” I/OMaster requests Input values from slave and sends Outputs on each bus cycle“Cyclic” I/OSlave sends data at predefined time intervalsChange of State (“COS”)Slave sends data only when a change of value has been detected.Bit StrobeA very efficient way of requesting a small amount of data (less than 4 16 bit words) from a slave device.Most often used method is Polled I/O - others useful toincrease throughput
7 DeviceNet “Objects” DeviceNet devices made up of a set of “Objects” Really just a way of organising parameters by functionsValues are addressed over comms by:“Class” - a number defining the type of object from a predefined list (e.g. Class 1 is a “DeviceNet identity” object holding information on Device model and manufacturer.)“Instance” - If more than one object of a given class exists, this number identifies a particular instance.“Attribute” - a number between 0 and 199 defining the parameter within the object.Objects accessed using “explicit messaging”.
8 Explicit MessagingLike Modbus, i.e. a single shot read or write request.Read or write an object by class/instance/attribute.Typically used by Configuration Tool to configure or commission a slave deviceCan also be used by a PLC master to access infrequently used variables in a slaveUsed in 2600 and 2700 to write program data
9 “EDS” files and Commissioning A bit like Profibus GSD files - define device capabilitiesHowever No “GSD file editor” equivalent in DeviceNet - Eurotherm slaves configured using standard DeviceNet tools.
10 DeviceNet v. Profibus DeviceNet Profibus DP Max Speed 12 Mbaud Max speed 500 kbaud125 kbaud250 kbaud500 kbaudMax Nodes 64 per networkPower supplied by networkComplex and extendable architectureFlexible trunk/drop topologySelf Declaration of NodesProfibus DPMax Speed 12 Mbaud500 kbaud1.5 mbaud12 mbaudMax Nodes 32 per segment, 128 per network (with repeaters.No Power on networkSimple, fixed architectureTrunk only topologyFixed network configurationProfibus DP is generally simpler to use and more capable. The only real practical advantage to DeviceNet is when linking into Allen Bradley/Omron systems
11 2600/2700 on DeviceNetFully Configurable I/O Messaging0 to 60 “Input” variables (from PLC)0 to 60 “Outputs” (sent to PLC)Polled/Cyclic/COS/Bit Strobe availableConfigured via an ‘Assembly Configuration Object” using standard Rockwell toolsSupport for 32 bit (IEEE floats and long integers)“Application” ObjectAllows explicit messaging access to 200 frequently used variablesIncludes 32 configurable ‘attributes’ assignable to any instrument parameter“Tag Access” objectPermits Modbus style ‘block’ read and writes.Used for program download and access to infrequently used variables.Currently under Beta test for release around April
12 2400 on DeviceNet Same as 26/2700, except: Only 10 configurable parameters in “Application Object”No support for IEEE numbersAvailable for beta test: release scheduled for May/June.
13 2500 on DeviceNetAs 2600/2700 except ALL 200 parameters in the application object are configurableBeta test units available: release scheduled for April/May
14 Some closing remarksWith the release of the new DeviceNet interface, Eurotherm will have a very complete and scalable DeviceNet offering, from 2200 up through 2700.Should be possible to repeat success of Profibus with DeviceNetConsider targeting Allen Bradley and Omron OEMsConsider what further training you would like!